Security was stepped up on London's public transport network today, as police shot a man dead on the underground a day after four attempted suicide bombings on trains and a bus.

Today's shooting occurred at Stockwell tube station in south London at 10am, as people returned to the capital's disrupted transport system after yesterday's failed attacks.

Police were out in force on London's tube and bus network, with sniffer dogs deployed in underground stations.

The Northern and Victoria lines were temporarily closed after today's shooting, adding to delays caused by existing closures in force since the bomb attacks on July 7 [for lastest information about disruption to London's transport system, click here].

There were a number of security alerts, including an incident on the Northern line and one at Vauxhall station. All of the alerts received the "all clear".

A police source said: "There have been at least six alerts in the last couple of hours from a suspect package, or something suspicious, that have been given the all clear. People are jumpy this morning."

Nigel Holness, Transport for London's service director, told BBC Radio 4's Today programme: "We have the maximum deployment possible of the British Transport police. We have started to use sniffer dogs around the underground. And we are looking at deploying staff in different ways."

Mr Holness said he expected more people to be searched on the tube system following the latest incidents but the scale of passengers meant searching everyone would be impossible.

He said using metal detectors and bag x-rays would be hugely disruptive, with 3 million journeys a day on the tube, including half a million at peak times. "It is impossible to search and scan those people," he said.

Some passengers were enduring difficult and anxious journeys today, but Mr Holness insisted good services were running on unaffected tube lines and bus services were operating as normal.

The underground stations involved in yesterday's incidents remained closed: Warren Street, in central London, and Oval, in south London, where there were small blasts on trains, and Shepherd's Bush, in west London, where a device was found.

Another device was found on a No 26 bus in Shoreditch. Some roads around the scenes remained closed today as police continued to collect forensic evidence.

The near-simultaneous, minor explosions at around 12.30pm yesterday, which police said were failed attempts to kill, caused chaos in the transport system for the second time in a fortnight and five tube lines were suspended.

The failed attacks, which injured one person, compounded the transport problems left by the suicide bombings two weeks ago that murdered 52 people. Some parts of the tube were still closed following that attack.

In the immediate aftermath of yesterday's attacks the Metropolitan police commissioner, Sir Ian Blair, told people to stay where they were, but in mid afternoon he said it was time to get the capital moving again.